Nonvolatile memory arrays maintain their data even when power to the device is turned off. In one-time-programmable arrays, each memory cell is formed in an initial unprogrammed state, and can be converted to a programmed state. This change is permanent, and such cells are not erasable. In other types of memories, the memory cells are erasable, and can be rewritten many times. A resistivity switching element is typically used for the programmable or storage element of the memory cell.
The resistivity switching element has more than one stable resistivity which depends on the crystal structure of the material of which it is made. A resistivity of the resistivity switching material can be changed, for example, by applying a voltage above a predetermined threshold voltage, thereby changing the crystal structure or forming a conductive link through a dielectric. Some resistivity materials are reversible. That is, application of a voltage above a second predetermined threshold results in the material returning to its previous crystal structure. Still other materials have more than two stable crystal structures and can provide more than two resistivity states.
Three dimensional switching memories use a selector device (steering element) in series with each resistivity switching element. The selector device reduces undesired leakage current which may flow through half-selected and unselected cells in the array when voltage is applied to a selected cell. Conventional three dimensional switching memories are configured to use unipolar switching. For unipolar switching, the selector device may be unidirectional, for example a diode.